[PATCH 6.2 055/110] Revert "userfaultfd: dont fail on unrecognized features"

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Peter Xu <peterx@xxxxxxxxxx>

commit 2ff559f31a5d50c31a3f9d849f8af90dc36c7105 upstream.

This is a proposal to revert commit 914eedcb9ba0ff53c33808.

I found this when writing a simple UFFDIO_API test to be the first unit
test in this set.  Two things breaks with the commit:

  - UFFDIO_API check was lost and missing.  According to man page, the
  kernel should reject ioctl(UFFDIO_API) if uffdio_api.api != 0xaa.  This
  check is needed if the api version will be extended in the future, or
  user app won't be able to identify which is a new kernel.

  - Feature flags checks were removed, which means UFFDIO_API with a
  feature that does not exist will also succeed.  According to the man
  page, we should (and it makes sense) to reject ioctl(UFFDIO_API) if
  unknown features passed in.

Link: https://lore.kernel.org/r/20220722201513.1624158-1-axelrasmussen@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20230412163922.327282-2-peterx@xxxxxxxxxx
Fixes: 914eedcb9ba0 ("userfaultfd: don't fail on unrecognized features")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Dmitry Safonov <0x7f454c46@xxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Cc: Zach O'Keefe <zokeefe@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/userfaultfd.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1966,8 +1966,10 @@ static int userfaultfd_api(struct userfa
 	ret = -EFAULT;
 	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
 		goto out;
-	/* Ignore unsupported features (userspace built against newer kernel) */
-	features = uffdio_api.features & UFFD_API_FEATURES;
+	features = uffdio_api.features;
+	ret = -EINVAL;
+	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES))
+		goto err_out;
 	ret = -EPERM;
 	if ((features & UFFD_FEATURE_EVENT_FORK) && !capable(CAP_SYS_PTRACE))
 		goto err_out;